Conventional process orchestration (also called a process plan) for assigning tasks to the respective processes (task units) on an assembly line for assembling products is optimized by taking into account compliance with the task order in each process and levelling of workloads. In a case where a robot is included in the assembly line, the time required for each task needs to be estimated, to calculate the cycle time (the time required for one task cycle) of the robot. The time required for each task is a total of the time required till a part is picked up at the part supply position, the time required till the picked-up part is conveyed to the assembling position (the transfer time for the robot), and the time required for incorporating the part into the product.